Implantable Medical Devices (IMDs) for producing a therapeutic result in a patient are well known. Examples of such IMDs include, but are not limited to, implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Such IMDs may treat a variety of symptoms or conditions including, but not limited to, chronic pain, migraine headaches, tremor, Parkinson's disease, epilepsy, incontinence, gastroparesis, heart failure, tachycardia, and bradycardia.
A common element in all of these IMDs is the need for electrical power in the device. The IMD requires electrical power to perform its therapeutic function, which may include driving an electrical infusion pump, providing an electrical neurostimulation pulse and/or providing an electrical cardiac stimulation pulse, for example.
Typically, a power source for an IMD can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires which perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power source for therapy is a large inconvenience.
A second type of power source utilizes single cell batteries as the energy source of the IMD. This can be effective for low-power applications, such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform new therapies in newer IMDs. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all, or nearly all, of its energy in less than a year. This is not desirable due to the need to explant and re-implant the IMD or a portion of the device.
One way to address the aforementioned limitations involves transcutaneously transferring electrical power through the use of inductive coupling. Such electrical power may then be optionally stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the IMD. When the battery has expended, or nearly expended, its capacity, the battery may be recharged. This is accomplished transcutaneously using electromagnetic coupling from an external power source that is temporarily positioned on the surface of the skin. Most often this will involve inductive coupling, but could include other types of electromagnetic coupling such as RF coupling.
Transcutaneous energy transfer through the use of electromagnetic coupling generally involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. An internal, or “secondary”, coil is part of, or otherwise electrically associated with, the IMD. An external, or “primary”, coil is associated with the external power source, or recharging device. The recharging device drives the primary coil with an alternating current. This induces a current in the secondary coil through inductive coupling. This current can then be used to power the IMD and/or to charge, or recharge, an internal power source.
For IMDs, the efficiency at which energy is transcutaneously transferred is crucial for several reasons. First, the inductive coupling has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. By increasing the efficiency of the energy transfer between the primary and secondary coils, heating of the tissue is minimized. Moreover, the time required to complete the recharge session is minimized, thereby maximizing patient convenience. Finally, if more energy may be transferred in a shorter period of time, IMDs may be employed that have higher power requirements and that provide greater therapeutic advantage to the patient.
One way to increase energy efficiency is to position the primary coil optimally with respect to the secondary coil. This generally involves positioning the primary coil on the patient's body (e.g., on their skin) as close to the secondary coil as possible. Moreover, the primary coil optimally lies in a plane that is parallel to the plane occupied by the secondary coil within the patient's body. This configuration is readily achieved in an implant scenario wherein the coil is implanted at a depth of between 1 and 3 centimeters in an orientation such that the IMD is positioned roughly parallel to the cutaneous boundary. This type of scenario may be used when an IMD is positioned within the pectoral region, as will be the case if the device is to be used to deliver electrical stimulation to areas of the brain, for instance.
In some cases, an IMD may be implanted more deeply within a patient's body. For instance, when an IMD is used to deliver therapy related to sacral nerve stimulation (SNS) as may be performed to treat incontinence, the IMD may be implanted more deeply within the abdominal cavity. When so implanted, the IMD may not be parallel to any particular cutaneous boundary, and in fact, may actually be perpendicular to such boundaries. As a result, less efficient recharge coupling is achieved, requiring longer recharge sessions.